TOPSAR: Novel Radar Technique
Sentinel-1 uses a cutting-edge SAR technique known as Terrain Observation with Progressive Scans SAR (TOPSAR).
As this video illustrates, like standard ScanSAR, TOPSAR sends and returns radar data to and from sub-swaths that together comprise one wide swath. The difference is that TOPSAR’s radar also scans back and forth along individual sub-swaths in the azimuth direction — or the direction the satellite is moving — and it does so more rapidly than the satellite itself is traveling. Video credit: ESA.
TOPSAR enables the Extra Wide Swath (EW) and Interferometric Wide Swath (IW) modes and facilitates interferometric SAR. In these modes, bursts are synchronised from pass to pass to ensure the alignment of interferometric pairs. Interferometry uses more than one image of the same location to detect motion such as land deformation. Examples include studies of volcanoes, earthquakes, and sinkholes.
Intended to replace ScanSAR
TOPSAR mode is intended to replace the conventional ScanSAR mode, achieving the same coverage and resolution as ScanSAR, but with a nearly uniform Signal-to-Noise Ratio (SNR) and Distributed Target Ambiguity Ratio (DTAR).
Azimuth resolution is reduced compared to ScanSAR mode due to the shorter target illumination time of the burst. Using the sweeping azimuth pattern, each target is seen under the same antenna pattern, independently from its azimuth position in the burst image. By shrinking the azimuth antenna pattern, as seen by a target on the ground, scalloping effects on the image can be reduced. Bursts are synchronized from pass to pass to ensure the alignment of interferometric pairs.
For TOPSAR, the processing must handle the antenna steering rate and the DC rate due to the steering. The azimuth pre- and post-processing of the data must include deramping of the data prior to base-band DC estimation, azimuth ambiguity estimation, and GRD azimuth processing. Please see the technical note COPE-GSEG-EOPG-TN-14-0025 for details on how deramping is performed by the IPF.
By rapidly scanning sections of all sub-swaths in the direction the satellite is traveling, TOPSAR yields more consistent image quality. Figure: De Zan, F., & Guarnieri, A. M. (2006).
TOPSAR azimuth antenna sweeping causes Doppler centroid variations of approximately 5.5 kHz introducing an azimuth phase ramp (azimuth fringes) for small co-registration errors. To correct this, azimuth co-registration is required to be better than 0.001 samples (pixels) in order to obtain phase error less than 3°.
To be useful for generating interferograms, TOPSAR bursts are synchronized between repeat-pass data takes. A burst synchronization of <5 ms is required.
Content on ASF’s Sentinel web pages is adapted from the European Space Agency (ESA) Sentinel Website.